Vapor-phase nucleation of n-pentane, n-hexane, and n-heptane: Critical cluster properties

The first and second nucleation theorems provide a way to determine the molecular content and excess internal energies of critical clusters, which rely solely on experimental nucleation rates measured at constant temperatures and supersaturations, respectively. Here, we report the size n* and excess internal energy Ex(n*) of n-pentane, n-hexane, and n-heptane critical clusters when particles form under the highly supersaturated conditions present in supersonic expansions. In summary, critical clusters contain from similar to 2 to similar to 11 molecules and exhibit the expected increase in the critical cluster size with increasing temperature and decreasing supersaturation. Surprisingly, the n* values for all three alkanes appear to lie along a single line when plotted as a function of supersaturation. Within the framework of the capillarity approximation, the excess internal energies determined for the n-heptane critical clusters formed under the low temperature (similar to 150 K) conditions in our supersonic nozzle are reasonably consistent with those determined under higher temperature (similar to 250 K) conditions in the thermal diffusion cloud chamber by Rudek et al. Published under license by AIP Publishing.